B) 'Technically Dead' status message and de-facto being unable to influence anything counts as dead in my book, regardless of whether you are a plant-man or not.
His status is Kelpman now, just in point of fact.
Now, for my two bits, I'm starting to agree a little with HMH, but for different reasons. Right now I'd rather not have Lamar killed, but I doubt I'd try and stop it if HMH is really intent. My reasoning for keeping him alive is this,
1. I admit that Lamar has been a complete health sink, hell, I don't remember him ever using fire magic without burning himself. And when I managed to start killing everyone with a giant plant, well, things went all pear shaped from there. Over the past several turns Lamar's condition has managed to wound, mortally or otherwise, every single person who has attempted to help him, but we are now at a point where he
"might" come back. The plant transformation is stabilized, and he is regenerating very rapidly.
In short, you kill him now, every single action that we've made to help him will have been in vain. That's a lot of actions, not to mention a lot of HP, that will have been lost pointlessly. I would rather not have that happen when our actions are so close to, perhaps literal, fruition.
2. It is entirely possible that killing him will merely remove the "man" suffix from his kelpman status. The plant obviously does not require him to be alive to grow, so chopping off his head might only remove the best chance to actually control this particular monstrosity. The brain thing that HMH has going on is unfortunate, but as yet there have been no negative effects. An attack on the host body might change that. It might be better to attempt to put the plant into a dormant state, something that
might be achieved as easily as putting Lamar's body in the spooky cave.
3. RP fracture. It's a silly reason, I know, but the action of killing Lamar would create a bit of a difficult situation, at least for Rhed, Greg, and Clarence. We would either have to sweep it under the great rug of meta, or end up with a battle royal over the fact that the Orc (Who doesn't like anyone, and isn't exactly popular himself) just murdered one of the group in cold blood. That situation would only further serve to exacerbate the wastefulness of killing Lamar, an action that, as noted above, is already an ugly proposition.
Of course, in counter to the above, I do realize that it may be (and probably is) very much Eric's character to murder Lamar for screwething up his mind.
Now, my two large arguments for killing Lamar and having done with it,
1. This will only happen again. Lamar is a fire mage in a world where mages are the equivalent of thirteen pitch-black cats in a shop full of precariously balanced mirrors. If he lives he is going to burn himself, or one of us, that's just the rule of the dice. The fact that he's a fire mage, and thus particularly damaging, makes this effect far more dangerous than it is for, say, an earth mage like Eric. (E.g: Eric fails a complicated earth spell: Gets entombed in earth for two turns. Lamar fails a simple fire spell: sets own hands on fire.) The consequences for future pyromantic catastrophe will be similar to the situation we have now, with the dice again deciding how far the swath of chain-reacting destruction will spread.
2. Lamar's corpse is a goldmine. Necromantic reagents for Eric and Gustavo, alchemical ingredients for Clarence, and biology training for Greg. The only person who wouldn't benefit would be Rhed, although he too might be able to train some with Greg. We were cheated out of the spoils of our last battle, the corpses and equipment that might have helped us survive. Metaphorically cannibalizing the dead might be the best option for our continued survival. Though that option is horribly Machiavellian and, as noted above, victim to a serious RP fracture.